a) Field of the Invention
The present invention relates to a waist level finder optical system which is to be used with lens shutter cameras and so on.
b) Description of the Prior Art
The conventional waist level finder optical system, typically exemplified by the waist level finder optical system proposed by Japanese Patent Preliminary Publication No. Sho 63-262635, is composed of an objective lens having positive refractive power and a field lens having positive refractive power which are arranged in order from the object side.
This conventional waist level finder optical system, i.e., an optical system which is composed of an objective lens having positive refractive power and a field lens which is arranged in the vicinity of an image point of the objective lens and has positive refractive power, will be examined according to the paraxial theory while regarding these lenses as thin lenses as illustrated in FIG. 1. In this drawing, the reference numeral 1 represents the objective lens, the reference numeral 2 designates the field lens and the following formulae apply to this optical system: EQU f.sub.1 =D (1) EQU 1/f.sub.1 =tan .omega./h (2) EQU 1/f.sub.2= 1/Le+1/D (3)
wherein the reference symbol D represents a total length of the waist level finder optical system, the reference symbol f.sub.1 designates a focal length of the objective lens, the reference symbol .omega. denotes a half angle of incidence of the waist level finder optical system, the reference symbol h represents a height, as measured on the field lens 2, of a principal ray incident at the half angle of incidence .omega. on the optical system, the reference symbol f.sub.2 designates a focal length of the field lens and the reference symbol Le denotes a distance as measured from the field lens 2 to an eye point.
The formula (1) means that the total length of the waist level finder optical system is equal to the focal length f.sub.1 of the objective lens when considered according to the paraxial theory. The formula (2) means that the focal length f.sub.1 of the objective lens 1 is determined dependently on the half angle of incidence .omega. and the height h of the principal ray. The formula (3) means that the focal length f.sub.2 of the field lens 2 is determined dependently on the distance Le and the total length D of the finder optical system. Accordingly, it is apparent from the above-mentioned formulae (1) and (2) that the total length D of the waist level finder optical system is determined once the half angle of incidence .omega. of the waist level finder optical system and the height of ray on the field lens 2 are decided.
In order to shorten the total length D of the waist level finder optical system with the half angle of incidence .omega. kept definite, however, it is obliged to lower the height h of the principal ray on the field lens 2, or reduce size of an image. As a result, the waist level finder optical system has a long distance as measured from the field lens to the eye point, thereby being inconvenient for observation of the image.